The invention relates to an exposure system for exposing a resist film on a semiconductor wafer, and further relates to an illuminating apparatus used in the exposure system for illuminating a mask having a circuit pattern to expose the resist film. The invention also relates to a method of the exposure.
Exposure process is one of the most very important processes for fabricating semiconductor integrated circuits. To achieve a scaling down of the semiconductor integrated circuits, it is essential that a precise and exact projection of a fine mask pattern to a resist film on a wafer is accomplished by use of an excellent exposure system.
A degree of the flatness of a surface of a wafer on which resist film is provided had substantially provided a limit to the critical dimensions in photo-lithography. The exposure is accomplished for projection of a mask pattern to a resist film having a photo-sensitivity provided on wafer wherein the mask is placed between an illumination source and the resist film. The mask comprises a transparent plate and opaque portions provided on a surface of the plate. Light for illumination passes through the mask except for the opaque portions.
Generally, the illumination light which has passed through the mask with the circuit pattern are irradiated at a right angle on a surface of the wafer on which the resist film with a photo-sensitivity was provided. The illumination light is irradiated on the resist film and passes it to arrive at the surface of the wafer where a part of the illumination light is then reflected. A reflective direction of the illumination light from the surface of the wafer depends upon an orientation of the surface of the wafer. Even ideal wafer has a completely flat surface, actual wafer necessarily has a somewhat irregular surface as including a sloped area or a dropped portion due to complicated multilayer structures involving wiring layers and any elements. When the illumination light is spotted on the sloped area, then a reflected portion of the light in the resist film on the wafer has an oblique direction. For that reason, the illumination light including a portion thereof once reflected from the sloped area has already lost the information of the mask pattern. The oblique reflection of the light by the oblique surface allows the reflected light portion to come into a position in the resist film however right under the opaque portion of the mask, even in the right under portion of the resist film no illumination light should exist. Namely, the a portion of the resist film, which should be kept from the exposure, is forced to be exposed to the illumination light. Further, even if the surface of the wafer has a good flatness, a large reflectivity of the illumination light may allow a standing wave to exist in the resist film thereby resulting in a serious problem with decline of the resist profile.
To combat the above problem, it was proposed to provide an anti-reflecting coat between the resist film and the surface of the wafer for suppression of the reflection of the light. This way is, however, engaged with the another serious problem in providing contamination to the wafer. In view of the fabrication processes, that way has a disadvantage in need for further fabrication steps.
Alternatively, proposed another way is to use a resist containing dye additives to reduce an amount of the illumination light capable of arriving on the surface of the wafer. Notwithstanding, this way is faced directly to other problem in decline of a vertical resist profile after exposure.